Two of the greatest figures in Victorian architecture and design were sons of Frenchmen. The first was Isambard Kingdom Brunel (who looked forward with clarity). The second was Augustus Welby Northmore Pugin (who looked back with sentiment), although both were in truth gloriously idiosyncratic Victorian personalities, monuments to their own age and no other.

Marc Brunel, the great railway engineer's father, was inspired by a marine mollusc to invent a prototype of the TBM (tunnel boring machine) that eventually made the Channel Tunnel a reality. In the days before the tunnel, Auguste Pugin, the architect's Protestant father, is said to have rowed across the Channel to join the bohemian community in Soho. Here he became an assistant to John Nash, ran a drawing school, published architectural illustrations and liked to be known as the Comte de Pugin. The young Brunel had a synoptic genius for the technical possibilities of engineering. The young Pugin, intoxicated by the Rites of Old Sarum, by Latin and crockets, rich colours, bells and smells, wanted Roman Catholicism to inspire English architecture and to purge the age of its secular miseries. This was his lasciviously polychrome and gorgeously scented version of the Gothic revival.

Rosemary Hill has written a superb study of this true romantic and tragic original. It is scholarly, but intimate, warm and readable too, immediately becoming the standard work.

Pugin's conversion to Catholicism came in 1835. Two years before, a sermon by John Keble had started the Oxford Movement. One year before, the Houses of Parliament burnt to the ground, an event captured in a magnificent painting by Turner (now in the Philadelphia Museum of Art). The competition to rebuild the ravaged Westminster stipulated a Gothic design, but out of 97 entries it was won by the classicist Charles Barry, who produced a clever plan and provided Gothic costume for what is in all essentials an Italianate formal concept. In 1844 Barry put Pugin, who had helped him with King Edward VI School in Birmingham, in charge of the interiors. The glorious colours, linen-fold, brass, tiles, wallpaper that resulted are all Pugin's. Even to the Modernist critic Ian Nairn, this floriated ornamental splendour was magnificent stuff. 'There are many ways to heaven and this is one of them,' he wrote. Pugin's hand can be seen as well in the profile and details of Big Ben, rivalled only by the Eiffel Tower as the planet's most familiar landmark.

Other architectural designs by Pugin included convents and presbyteries and rectories. His own home at Ramsgate of 1844 is his domestic masterpiece: an exemplary model for imitation, entirely free from Georgian tweeness, and full of butch common sense, even rationality in its floor plans, a prototype for thousands of imitations. Additionally, he was a brilliant designer of ornament and a completely original colorist. Nor was he afraid of being clear-sighted and contrarian. The Wye Valley in Pugin's day was popular for picturesque tourism, but he found Tintern Abbey 'disappointing'. In 1846 his style reached its sumptuous crescendo in St Giles, Cheadle, and in 1851 he showed his own 'Medieval Court' at the Great Exhibition of the Works of Industry of All Nations. In an exhibition dedicated to manufacturing, this was a landmark moment in the emergence of the designer as an independent personality, although controversial in its introduction of a 'Popish chapel' into an event whose purpose was not metaphysical, but to sell steam pumps to colonials.

Pugin was an inspired designer but was most influential as a pamphleteer. It was his campaigning that allowed Nikolaus Pevsner and others to claim him for Modernism. His credo was set out in Contrasts (1836) where his father's employer, John Nash, architect of the Potemkin village that is Regent's Park, is ridiculed on the title-page. Pugin demonstrates the Godless reality of early Victorian life by contrasting Bentham's utilitarian panopticon (with corpses being carried off for dissection) with a fine medieval cathedral (and de luxe Christian burial). Next, in 1841, came The True Principles of Pointed or Christian Architecture. Here Pugin criticised 'absurdities' in design that 'proceed from the false notion of disguising instead of beautifying articles of utility'.

Classicism was always stigmatised: 'What madness ... to worship at the revived shrines of ancient corruption,' he unambiguously declared. Passing Parliament on the Thames, he was heard to mutter: 'Greek.'

Although Ruskin shared these views, protesting too much, he denied any influence from Pugin. But it was when William Morris brought Pugin's art criticism together with Ruskin's social criticism that the cultural conditions for the Modern movement were established. Morris believed: 'It is not possible to dissociate art from morality, politics and religion.' Pugin's Gothic revival was really nothing of the sort: it was inventive novelty inspired by the notion that 'good' architecture and design can enhance life just as 'bad' architecture and design can diminish it. Pugin has a surprising amount in common with Terence Conran and James Dyson, not to mention Walter Gropius.

This insistence on 'honesty' and 'social purpose' fed the Arts and Crafts of the 1880s and 1890s, and the Bauhaus in the 1920s. But Pugin saw none of it. He died aged 40, insane in his own house. Leeches had been applied but could not deter his decline. Today Pugin would be diagnosed as bipolar. The architect GG Scott found him 'tremendously jolly', but he was also given to weeping in public and his short life was punctuated by frequent bouts of private melancholy, personal tragedy and psycho-sexual upheaval. For some time before his death his eyes were fixed, perhaps contemplating the vast hallucination of patterns and ideas he created.

God's Architect is more a cultural history than strict architectural history, but none the worse for that. Anyway, for Pugin, architecture was not separate from culture, but the most powerful expression of it. And the motto of God's architect? 'En avant.' Or what might be freely translated as 'go for it'. Go for it, he did.

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